Packet switching networks are being used in more and more applications to provide communications among distributed processors, wherein the sum total of the processors can provide more processing power than any one large processor. Increasingly, these processing networks communicate with other processing networks to further distribute the processing load, and for other reasons. Such inter-network communication is provided by a "gateway" in each system whose job it is to send and receive messages over the communications media connecting it to another gateway, and to perform any translation of protocol necessary for transmission on the gateway's network (referred to as the "home" network).
Today's packet switching networks continually increase in network speed as new technology becomes available. However, gateways generally have a throughput bandwidth (i.e., the speed at which the gateway can perform the protocol and/or data format changes) less than the speed of its home network. Further, the speed of the interconnection medium is frequently far less than the speed of the network. As a result, if there are many messages going from the network to the gateway, the gateway becomes overloaded.
In the prior art, the problem of gateway overload is usually addressed by providing the gateway with a very large receive buffer, by the gateway dropping messages when it becomes overloaded, or both. Gateways with very large receive buffers are expensive both in cost of the buffer and the system needed to maintain such a large buffer. Dropping messages is generally an acceptable solution only for those applications that use a level two or level three protocol. In these protocols, the sender waits for an acknowledgment back from the receiver, otherwise the sender sends the message again. Such solutions are appropriate for data or low speed applications, but not for high speed data transmission or low latency applications, such as packetized voice. Handshake protocols not only slow transmission down, but in many applications, the acknowledgment and retransmission protocols cannot be used, due to the low latency requirements. In these applications, dropping messages may be catastrophic.
FIG. 1 illustrates an example of packetized voice transmission which requires such low latency. FIG. 1 is a block diagram of a code division multiple access (CDMA) wireless telephone switching system covering a wide geographical area. A CDMA system transmits/receives voice or data at the relatively slow rate of approximate 8 Kbps between a mobile unit 54 and a cell site such as 39 over a spread spectrum signal. A transceiver at the cell site sends/receives the spread spectrum signal and translates the content of the signal into packets. A speech processor (SP) then translates the packets into a digital 64 Kbps pulse code modulated (PCM), as used in standard digital switching. CDMA cellular communication is more fully described in "The Wideband Spread Spectrum Digital Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard," "CDMA Digital Common Error Interface Standard," revision 1.0, October, 1990, and "An Overview of the Application of Code Division Multiple Access to Digital Cellular Systems and Personal Cellular Networks," May 21, 1992, available from Qualcomm, Inc.--10555 Sorrento Valley Road--San Diego, Calif.
In most CDMA system designs, the speech processor is at the cell site. However, in this illustrated embodiment, the speech processor is on a packet switch, (such as 47, 147, and N47), connected to the cell site and the data received at the transceiver is packetized and then sent through a packet network to the designated speech processor. A system and method for such packetization and routing through the packet networks is described in U.S. patent applications Ser. Nos. 08/040,819 and 08/040,818, assigned to the assignee of this invention, which are incorporated herein by reference.
In this exemplary embodiment, wireless telephone (not shown) 50 in car 54 initiates a call to telephone 100, when car 54 is in cell 5, connected to packet switch 47. Packet handler (PH) 55 receives packets from cell 5 and sends them on bus 61 to speech processor (SP) 57, which connects the call to and from public switched telephone (PSTN) 3 and, thus, to telephone 100. As mobile 54 moves from cell 5 to cell 7, an executive call processor (EPC) network (not shown for clarity in this figure but well known in the art) informs cell site 7 of the SP 57 handling the call. As wireless telephone 54 moves into the boundary area between cells 7 and 9, packet handlers 53 and 54 both send packets on bus 61 to speech processor 57. Speech processor 57 continues to be the only connection to PSTN network 3 and, thus, to telephone 100 for this call. Wireless telephone 54 then moves fully into cell 9, and only packet handler 51 sends packets to speech processor 57.
Wireless telephone 54 then moves from cell 9 into adjoining cell 25, which is serviced by packet handler 155, and packet switch 147. Cell 25 sends packets to packet handler 155, which places them on packet bus 161. Gateway 202 recognizes that the address is not for a member of the packet bus 161 community, encapsulates the packets with ATM protocol, as described in the above-referenced patent applications, indicating the destination community, and sends them through self-routing asynchronous transfer mode (ATM) network 400. Self-routing ATM network 400 examines the address and routes the cells to the appropriate gateway, which in this case is gateway 200, since the cells are destined for packet bus 61 and speech processor 57. Encapsulated packets arrive at gateway 200 in packet switch 47, are reassembled, and put on packet bus 61 to speech processor 57.
Similarly, when wireless telephone 54 moves through cells 35, 37, 39, and all intervening cells, all packets are sent to/from speech processor 57, since all of the packet switches are connected to ATM network 400. Thus, the call from speech processor 57 through PSTN 3 to telephone 100 does not have to be torn down as wireless telephone 54 traverses cell and switch boundaries. Therefore, no hard hand-off ever takes place throughout the entire network. An entire metropolitan area may be connected in this manner, with all cellular switches connected to one ATM network.
In the above example, only one wireless telephone is shown. In reality, there are many wireless devices, all sending calls to speech processors which are not necessarily on the same packet switch. Therefore, more packet handlers send packets to speech processors that are not on the same packet switch and, thus, send more traffic through their respective gateways 200 and 202, than they send to a destination on their own packet bus. So, for example, if there are 50 speech processors in the packet switch network of FIG. 1 equally distributed among five switches, then only one fifth (on the average) of all packets will be handled in the same packet switch. That means four fifths of the packet traffic will be sent through gateways. Additionally, CDMA packet traffic is packetized voice samples which are very time sensitive. If these packets are not delivered at a nearly steady rate, the call will be torn down. Thus, gateways 200 and 202 must have some method to avoid being overloaded by heavy time-sensitive packet traffic being sent to other switches.
Therefore, a problem in the art is that there is no system and method for preventing gateways from becoming overloaded when the nodes on the gateway's network are sending many messages to other networks.